1. Field of the Invention
The present invention relates to a conductive adhesive used to mount an electronic component on a substrate or the like, and more particularly, to a conductive adhesive comprising an adhesive resin and a plurality of conductive particles mixed in the adhesive resin. The present invention also relates to a mounting structure formed using such a conductive adhesive. The present invention also relates to a liquid crystal device formed using such a mounting structure. The present invention also relates to an electronic device formed using such a liquid crystal device. The present invention also relates to a method of producing a mounting structure, a liquid crystal device, and an electronic device.
2. Description of the Related Art
Liquid crystal devices are widely used in various electronic devices such as a portable telephone, a portable electronic terminal, etc. In many cases, liquid crystal devices are used to display information in the form of a character, a numeral, or a graphical image.
In general, a liquid crystal device includes a pair of liquid crystal substrates and a liquid crystal disposed between the pair of liquid crystal substrates wherein an electrode is formed on the inner surface of each substrate. The alignment of the liquid crystal is controlled by varying the voltage applied across the liquid crystal thereby modulating light incident on the liquid crystal. To control the voltage applied to the liquid crystal, it is required to use a liquid crystal driving integrated circuit, that is, a semiconductor chip, which is connected to the liquid crystal substrate directly or indirectly via a mounting structure.
In the case where the liquid crystal driving integrated circuit is connected indirectly to the liquid crystal substrate via the mounting structure, the mounting structure is formed, for example, by mounting the liquid crystal driving integrated circuit on a base substrate on which an interconnection pattern and electrode terminals are formed, and the resultant mounting structure is connected to the substrate of the liquid crystal device. In this case, when the liquid crystal driving integrated circuit is mounted on the base substrate, the liquid crystal driving integrated circuit may be connected to the base substrate using a conductive adhesive such as an ACF (anisotropic conductive film). More specifically, the liquid crystal driving integrated circuit and the base substrate are adhesively fixed to each other using an adhesive resin contained in the ACF such that bumps or terminals of the liquid crystal driving integrated circuit are electrically connected to electrode terminals on the base substrate via conductive particles contained in the ACF.
In some cases, when a semiconductor chip such as a liquid crystal driving integrated circuit is mounted on a base substrate, a passive electronic component such as a capacitor or a resistor or other electronic components such as a connector are also mounted on the base substrate via solder or the like, separately from the liquid crystal driving integrated circuit. Soldering for this purpose is generally performed using a solder reflow technique.
In the solder reflow technique, a solder pattern is first formed on a base substrate, at a predetermined location, by means of printing, dispensing, or the like, and a chip component such as the passive electronic component is placed on the solder pattern. The base substrate with the chip component placed thereon is then placed in a high-temperature furnace so as to melt the solder thereby soldering the chip component to the base substrate. In this process, the temperature of the heating furnace is set within the range of 200xc2x0 C. to 250xc2x0 C. The base substrate is exposed in a high-temperature region in this heating furnace for a short time. After completion of heating, the base substrate is cooled.
The conventional conductive adhesives such as an ACF used to mount a semiconductor chip include conductive particles formed of a synthesis resin such as polyester which is poor in heat resistance. Therefore, it is impossible to perform solder reflow processing after mounting a semiconductor chip on the base substrate using a conventional conductive adhesive. Thus, in conventional techniques, soldering of a chip component is first performed, and then the semiconductor chip is mounted using an ACF or the like.
In view of the above, it is an object of the present invention to provide a conductive adhesive which can withstand solder reflow processing. It is another object of the present invention to provide a mounting structure using such a conductive adhesive, a liquid crystal device using such a mounting structure, an electronic device using such a liquid crystal device. It is still another object of the present invention to provide a method of producing a mounting structure, a liquid crystal device, and an electronic device, using a simplified process.
1. According to an aspect of the invention, to achieve the above objects, there is provided a conductive adhesive comprising an adhesive resin and a plurality of conductive particles mixed in the adhesive resin, wherein: (1) each conductive particle comprises a core formed of a synthetic resin and a conductive material covering the core; and (2) the synthetic resin forming the core has a thermal deformation temperature higher than the thermal deformation temperature of the adhesive resin.
In this conductive adhesive, as described above, cores of the respective conductive particles contained in the conductive adhesive are made of a synthetic resin so that when two objects to be connected are brought into contact with each other via the conductive particles, the conductive particles are elastically deformed to a proper degree thereby achieving a stable contact between the two objects and thus achieving a stable electric connection between the two objects.
Furthermore, because the cores of the respective conductive particles contained in the conductive adhesive are formed using a synthetic resin having a high thermal deformation temperature, the conductive adhesive has high heat resistance. As a result, the conductive adhesive can withstand a solder reflow process in which the entire objects to be soldered are placed in a high temperature furnace. Therefore, it becomes possible to solder a chip component such as a capacitor to a base substrate by means of solder reflow processing after mounting a semiconductor chip on the base substrate using the conductive adhesive.
In the conductive adhesive described above, the adhesive resin may be a thermosetting resin such as an epoxy resin, a urethane resin, or an acrylic resin, wherein they have a thermal deformation temperature about 100xc2x0 C. below which no deformation occurs. The conductive material covering the cores of the conductive adhesive may be, for example, nickel or carbon.
2. In the conductive adhesive described above, the synthetic resin forming the cores preferably has a thermal deformation temperature (18.6 kg/cm2) equal to or higher than 120xc2x0 C. as measured according to the measuring method defined in the ASTM (American Society of Testing Materials) standard D648. This allows the conductive adhesive to have higher heat resistance which ensures that the conductive adhesive withstands solder reflow processing.
3. The synthetic resin having a thermal deformation temperature higher than 120xc2x0 C. as measured according to the ASTM-D648 standard may be selected from the group consisting of polyphenylene oxide, polysulfone, polycarbonate, polyacetal, and polyethylene terephthalate. If the cores of the conductive adhesive are formed using one of the synthetic resins described above, the conductive adhesive has high heat resistance which allows the conductive adhesive to withstand solder reflow processing. The characteristics of the respective synthetic resins described above are summarized in Table 1.
In Table 1, the respective characteristics are measured according to the standards described below:
Thermal Deformation Temperature: ASTM D648
Coefficient of Linear Expansion: ASTM D696
Specific Gravity: ASTM D792
Tensile Strength: ASTM D638
4. According to another aspect of the present invention, there is provided a mounting structure comprising a base substrate and a component mounted on the base substrate via a conductive adhesive, wherein: (1) the conductive adhesive is formed of an adhesive resin and a plurality of conductive particles mixed in the adhesive resin; (2) each conductive particle includes a core formed of a synthetic resin and a conductive material covering the core; and (3) the synthetic resin forming the core has a thermal deformation temperature higher than the thermal deformation temperature of the adhesive resin.
If this mounting structure is employed, it is possible to first mount a semiconductor chip on a base substrate using the conductive adhesive and then mount a chip component on the base substrate by means of solder reflow processing. Thus, the mounting structure is stable and has a high reliability.
The mounting structure may be produced in the form of a COB (chip on board) or a COF (chip on FPC). The mounting structure in the form of a COB may be produced by mounting a semiconductor chip or the like on a rather rigid and thick substrate such as an epoxy substrate. On the other hand, the mounting structure in the form of a COF may be produced by mounting a semiconductor or the like on a flexible and thin substrate such as a flexible printed circuit (FPC) substrate.
The flexible printed circuit substrate may be produced by forming an interconnection pattern using Cu or the like on a base layer made of polyimide or the like. If an interconnection pattern is directly formed on the base layer by means of a photolithography technique or the like, it is possible to achieve a two-layer FPC substrate. If an interconnection pattern is formed on the base layer via an adhesive layer, it is possible to a three-layer FPC substrate, wherein the adhesive layer serves as one of layers.
5. According to another aspect of the present invention, there is provided a liquid crystal device comprising: a liquid crystal panel including a liquid crystal disposed between a pair of substrates; and a mounting structure connected to the liquid crystal panel, wherein: (1) the mounting structure comprises a base substrate and a component mounted on the base substrate via a conductive adhesive; (2) the conductive adhesive is formed of an adhesive resin and a plurality of conductive particles mixed in the adhesive resin; (3) each conductive particle includes a core formed of a synthetic resin and a conductive material covering the core; and (4) the synthetic resin forming the core has a thermal deformation temperature higher than the thermal deformation temperature of the adhesive resin.
6. According to another aspect of the present invention, there is provided an electronic device including a mounting structure, wherein: (1) the mounting structure comprises a base substrate and a component mounted on the base substrate via a conductive adhesive; (2) the conductive adhesive is formed of an adhesive resin and a plurality of conductive particles mixed in the adhesive resin; (3) each conductive particle includes a core formed of a synthetic resin and a conductive material covering the core; and (4) the synthetic resin forming the core has a thermal deformation temperature higher than the thermal deformation temperature of the adhesive resin.
The electronic device may further include a liquid crystal device and a case in which the liquid crystal device is disposed, wherein the liquid crystal device comprises a liquid crystal panel including a liquid crystal disposed between a pair of substrates and wherein the liquid crystal panel is connected to the mounting structure.
7. According to another aspect of the present invention, there is provided a method of producing a mounting structure comprising a base substrate, a first component adhesively connected to the base substrate via a conductive adhesive, and a second component mounted on the base substrate by means of soldering, the method comprising the steps of: (1) mounting the first component on the base substrate via the conductive adhesive; and (2) mounting the second component on the base substrate by means of a solder reflow process after mounting the first component on the base substrate, wherein: (a) the conductive adhesive is formed of an adhesive resin and a plurality of conductive particles mixed in the adhesive resin; (b) each conductive particle includes a core formed of a synthetic resin and a conductive material covering the core; and (c) the synthetic resin forming the core has a thermal deformation temperature higher than the thermal deformation temperature of the adhesive resin.
In this method of producing a mounting structure, because the cores of the respective conductive particles contained in the conductive adhesive are formed using a synthetic resin having a high thermal deformation temperature, the conductive adhesive has high heat resistance. Therefore, when a chip component such as a capacitor is soldered to a base substrate by means of solder reflow processing after mounting a semiconductor chip on the base substrate using the conductive adhesive, the conductive adhesive can withstand exposure to a high temperature during the soldering process.
When a component is mounted using the solder reflow process, the process is influenced by contamination to a lesser degree than in the case where a component is mounted using a conductive adhesive such as an anisotropic conductive film, and does not require such high cleanliness level. Therefore, even if contamination occurs during a pre-process, the mounting process using the solder reflow technique hardly encounters problems. This results in suppression of occurrence of failures compared to the case where the mounting process using the conductive adhesive is performed after completion of the mounting process using the solder reflow technique.
8. According to another aspect of the present invention, there is provided a method of producing a liquid crystal device comprising: a liquid crystal panel including a liquid crystal disposed between a pair of substrates; and a mounting structure connected to said liquid crystal panel, the mounting structure comprising a base substrate, a first component adhesively connected to the base substrate via a conductive adhesive, and a second component mounted on the base substrate by means of soldering, the method comprising the steps of: (1) mounting the first component on the base substrate via the conductive adhesive; and (2) mounting the second component on the base substrate by means of a solder reflow process after mounting the first component on the base substrate, wherein: (a) the conductive adhesive is formed of an adhesive resin and a plurality of conductive particles mixed in the adhesive resin; (b) each conductive particle includes a core formed of a synthetic resin and a conductive material covering the core; and (c) the synthetic resin forming the core has a thermal deformation temperature higher than the thermal deformation temperature of the adhesive resin.
9. According to another aspect of the present invention, there is provided a method of producing an electronic device including a mounting structure comprising a base substrate, a first component adhesively connected to the base substrate via a conductive adhesive, and a second component mounted on the base substrate by means of soldering, the method comprising the steps of: (1) mounting the first component on the base substrate via the conductive adhesive of the present invention; and (2) mounting the second component on the base substrate by means of a solder reflow process after mounting the first component on the base substrate.